Processes for the conversion of coal and other hydrocarbons such as natural gas to a gaseous mixture consisting essentially of hydrogen and carbon monoxide, or of hydrogen and carbon dioxide, or of hydrogen and carbon monoxide and carbon dioxide, are well known. Although various processes may be employed for the gasification, those of major importance depend either on the partial combustion of the fuel with an oxygen-containing gas or on the high temperature reaction of the fuel with steam, or on a combination of these two reactions. An excellent summary of the art of gas manufacture is given in Encyclopedia of Chemical Technology, Edited by Kirk-Othmer, Second Edition, Volume 10, pages 353-433, (1966), Interscience Publishers, New York, N. Y., the contents of which are herein incorporated by reference. The techniques for gasification of coal or other solid, liquid or gaseous fuel are not per se considered part of the present invention.
It is known that synthesis gas can be converted to reduction products of carbon monoxide, such as hydrocarbons, at from about 150.degree. C. to about 450.degree. C., under from about- one to one thousand atmospheres pressure, over a fairly wide variety of catalysts. The Fischer-Tropsch process, for example, which has been most extensively studied, produces a range of liquid hydrocarbons, a portion of which have been used as low octane gasoline. Catalysts that have been studied for this and related processes include those based on iron, cobalt, nickel, ruthenium, thorium, rhodium and osmium, or their oxides. The wide range of catalysts and catalyst modifications disclosed in the art and an equally wide range of conversion conditions for the reduction of carbon monoxide by hydrogen provide some flexibility toward obtaining selected types of products, and some control over their molecular weight distribution Ruthenium catalyst, for example, is capable of producing linear hydrocarbons exclusively, while "promoted iron" also produces oxygenates. Nonetheless, these conversions still leave much to be desired because either the catalyst is costly or by-products are produced in excessive amount. A review of the status of this art is given in "Carbon Monoxide-Hydrogen Reactions", Encyclopedia of Chemical Technology, Edited by Kirk-Othmer, Second Edition, Volume 4 pp. 446-488, Interscience Publishers, New York, N. Y., the text of which is incorporated herein by reference for background.
The molecular weight distribution of the product in the Fischer Tropsch reaction is controlled to a great extent by the nature of the reaction, and it is generally recognized that the steady state products of the reaction follow the Schulz-Flory distribution. See, e.g., P. Biloen and W.M.H. Sachtler, Advance-in Catalysis, Vol. 30, pp. 169-171 (Academic Press, New York, N. Y., 1981), which is herein incorporated by reference for background. Very briefly, for this is well described elsewhere, if the synthesis that takes place is characterized by a stepwise addition of a single carbon species to a growing hydrocarbon chain with a propagation rate constant k.sub.p, and if this step competes with a growth-terminating step having the rate constant k.sub.t, then the chances for any intermediate species to propagate rather than terminate is described by , wherein EQU .alpha.=k.sub.p /(k.sub.p +k.sub.t)
If .alpha. is independent of the molecular weight of the intermediate, EQU log C.sub.n =Constant+n(log.alpha.)
where C.sub.n is the mole percent of the (n)th-mer in the product and n is the number of carbon atoms contained in that species. A plot of log C.sub.n vs n provides a straight line with the slope log.alpha..
The significance of the foregoing relationship for producing hydrocarbons by the Fischer Tropsch process is that a reduction of by-product methane formation also reduces larger amounts of C.sub.2, C.sub.3, and C.sub.4 hydrocarbons and causes a significant increase in the total yield of C.sub.5 +liquids, with more liquid in the diesel fuel range being formed.
In brief, when practitioners in the Fischer Tropsch art refer to the selectivity of a catalyst or process in terms of the relative amount of methane that is produced, it is generally understood in the context of the overall changes in the distribution of normally gaseous and liquid hydrocarbon product as outlined above. It is generally recognized in this art, however, that selectivity is a function not only of the catalyst composition and its method of preparation, but also is a function of process conditions, particularly temperature, and a function of synthesis gas composition. In general, a decrease in temperature results in improved selectivity for liquid hydrocarbons, and a similar result tends to be achieved with a synthesis gas that, within limits, is relatively rich in carbon monoxide. In principle, of course, selectivity for increased liquid hydrocarbons can be obtained by simply lowering temperature, but such an expedient also lowers conversion. As a practical matter, there is a lower temperature limit, dictated by the economically required conversion rate, below which operation becomes impractical.
Precipitated iron catalysts have been extensively studied and have been used for many years in the Fischer-Tropsch liquid phase process for synthesis of hydrocarbons. In general, they are inexpensive, exhibit good activity, and have adequate useful life. They almost always contain potassium promoter, which serves to reduce the amount of methane and other light hydrocarbon by-products. However, the amount of potassium that is normally used is limited to about 0.6 wt%, since larger amounts do not appear to offer further benefit with regard to methane reduction, and in fact increase the production of oxygenated by-products. Thus, there is a need for an iron catalyst having a higher selectivity for liquid hydrocarbons than is presently achieved in order to increase the total liquid hydrocarbons formed, especially those in the boiling range of high quality diesel fuel.
Conventional techniques for the production of a precipitated, inactive iron catalyst in large quantity and its activation prior to use are described by H. Koebel and M. Ralek, Catalysis Review-Sci Eng (1980) Volume 21, pp. 242-249, the entire content of which is incorporated herein by reference as if fully set forth. The initial steps in the preparation of the precipitated inactive iron catalyst useful in this invention are conventional. Ferric nitrate, which may be obtained by dissolving wrought iron scrap or steel turnings in nitric acid or, alternatively, from another source, is dissolved in water. The solution should be adjusted, if necessary, so that it contains a predetermined small amount of copper. The iron is then precipitated with ammonia or ammonium carbonate. Potassium carbonate is then added to the filtered and washed precipitate to provide a content of 0.1 to about 1.0 wt% potassium carbonate based on iron. The preferred potassium carbonate level is about 0.2 to 0.6 wt% based on iron content.
The filter cake produced by the technique just described and followed by the conventional step of calcining in air at e.g. 572.degree. F, usually contains well in excess of 1000 ppm (parts per million) of nitrogen. For certain special applications, an iron catalyst having a nitrogen content less than 200 ppm, preferably less than 100 ppm, may be needed. Such catalyst may be prepared by bringing together the ammonia solution and the ferric nitrate solution at controlled rates such that the pH of the cooled supernatant liquid containing the precipitated catalyst is maintained at about 6.8. The filter cake produced by this method is then washed with hot water until relatively free of nitrate ion. The resulting calcined filter cake produced by this technique is of low nitrogen content For further details, see U.S. 4,617,288 to BeI1 et al., incorporated herein by reference.
It is generally known that iron catalysts, as initially formed, are inactive in the Fischer Tropsch synthesis. Before use in the synthesis process, they must be subjected to an activation step which comprises contacting the inactive solid with a reducing gas, such as synthesis gas, at elevated temperature.
During activation, the iron is partially reduced to the metallic bonding state. This activation is conducted in the absence of water.
Water is known to be a powerful inhibitor in the Fischer Tropsch synthesis. Carbon dioxide is also an inhibitor, but very much weaker than water. The primary step in the conversion produces water by reaction (I): 2H.sub.2 +CO.fwdarw.--CH.sub.2 --+H.sub.2 O, (I) but much of the water is consumed by the shift reaction (II) catalyzed by the iron catalyst: EQU H.sub.2 O+CO .revreaction.H.sub.2 +CO.sub.2. (II)
To minimize the inhibiting effect of water, the synthesis gas feed to the Fischer Tropsch process and the recycle streams usually are dried prior to contact with the iron catalyst.
It is known that the catalyst life of an iron catalyst used in the Fischer-Tropsch process is limited by loss of activity during synthesis. For example, catalyst life of six months was reported for a fixed-bed unit that employed an alkaline, precipitated iron catalyst. During the six-month period, the operating temperature was increased from the starting point of 220.degree. C to a maximum of 255.degree. C. This temperature compensation for loss of activity certainly incurs a substantial loss of selectivity.
Koenig et al., in Ber. Bunsenges. Phys. Chem. 91, 116-21 (1987), report the results of an investigation of the influence of water and of alkali promoter on carbon number distribution of products formed over iron catalyst. U.S. 4,252,736 to Haag et al. discloses and claims adding water or steam to the Fischer-Tropsch conversion zone to adjust the H2/CO ratio of a syngas feed containing excessive hydrogen. Neither of these references disclose the present invention.
It is an object of this invention to provide a method for improving the selectivity of a used Fischer-Tropsch iron catalyst. It is a further object of this invention to provide a method for restoring the selectivity of a Fischer-Tropsch precipitated iron catalyst which has decayed during use. These and other objects will become apparent to one skilled in the art on reading this entire specification including appended claims.